The application of coatings to metals to retard or inhibit corrosion is well established, especially for metal food and beverage cans and other containers. Coatings are typically applied to the container interior to prevent the contents from contacting the metal of the container. Contact between the metal and the packaged product can lead to corrosion of the metal container, which can contaminate the packaged product. This is particularly true when the product is chemically aggressive. Protective coatings are also applied to the interior of food and beverage containers to prevent corrosion in the container headspace between the product fill line and the container lid, and is of particular importance for high-salt-content food products.
Packaging coatings preferably are capable of high-speed application to a substrate and following hardening preferably meet a variety of requirements for this demanding end use. For example, the coating should be safe for food contact; not adversely affect the taste of the packaged food or beverage product; have excellent adhesion to the substrate; resist staining and other coating defects such as “popping,” “blushing” or “blistering”; and resist degradation over long periods of time, even when exposed to harsh environments. In addition, the coating desirably maintains film integrity during container fabrication, withstands the various processing conditions that the container may be subjected to during product packaging, and withstands normal usage of the container (e.g., the coating is capable of maintaining its integrity when the can is dropped from a typical height).
A variety of coating compositions, including polyvinyl-chloride-based coatings, have been used to form interior protective can coatings. However, the recycling of materials containing polyvinyl chloride or related halide-containing vinyl polymers can be problematic.
Bisphenol A and bisphenol F monomers have been used to prepare polymers having a variety of properties useful for packaging. For example, bisphenol A or bisphenol F may be reacted with phosgene to provide polycarbonates that may be used to form packaging containers, and may be reacted with epichlorohydrin to provide packaging coatings. There is a desire to reduce or eliminate the use of certain bisphenol A-based and bisphenol F-based compounds in containers and coatings, and especially those involving contact with foods or beverages. There is also a desire to reduce or eliminate certain epoxy compounds commonly used to formulate food-contact coating compositions.
To address the aforementioned shortcomings, the packaging coatings industry has sought coatings based on alternative binder systems such as polyester resin systems. It has been problematic, however, to formulate polyester-based coatings that exhibit the required balance of coating characteristics (e.g., flexibility, adhesion, corrosion resistance, stability, resistance to crazing, etc.). For example, there has been a tradeoff between corrosion resistance and fabrication properties for such coatings. Polyester-based coatings suitable for food contact that have exhibited both good fabrication properties and an absence of crazing have also tended to be too soft and to have unsuitable corrosion resistance. Conversely, polyester-based coatings suitable for food contact that have exhibited good corrosion resistance have typically exhibited poor flexibility and unsuitable crazing when fabricated.
From the foregoing, it will be appreciated that what remains needed in the art are improved coating compositions for use in packaging coatings.